1. Field of the Invention
The present invention relates to optical media recording devices, and more particularly, to an optical media recording device capable of performing AACS encryption and related method for protecting device keys of the optical media recording device.
2. Description of the Prior Art
In the past, a variety of recordable media have been developed for use in recording and distributing commercial films, as well as home videos. Examples of such include video CDs (VCDs), VHS tapes, LaserDiscs, DVDs, and Blu-ray Discs (BDs). With the exception of VHS tapes and LaserDiscs, which may be considered analog media, modern media, such as DVDs and BDs, may be considered digital media. In other words, digitally encoded video data may be recorded to DVDs and BDs using an optical media recording device, and then read out, decoded, and played back using an optical media playback device.
One advantage of digital encoding over analog encoding is ability to encrypt the digitally encoded video data, which may prevent unauthorized copying and distribution of copyrighted media. A Content Scramble System (CSS) is employed as a Digital Rights Management (DRM) scheme for protecting commercially produced DVDs. CSS key sets are licensed for use in DVD burners and DVD players, and manufacturers desiring to make compliant devices are expected to follow all requirements of the CSS system. For example, all compliant disks incorporate keys written into a lead-in portion of the DVD, so a compliant DVD player must be able to read the keys in the lead-in portion to be able to play the rest of the DVD properly. However, as CSS employed a relatively weak encryption algorithm, with key lengths of around 25 bits, a brute-force algorithm was later developed which could crack the keys in the lead-in portion in under a minute. Thus, the CSS encryption system is easily compromised.
As new high-definition (HD) media types, such as the BD mentioned above, are developed, new encryption schemes are also employed. Blu-ray discs utilize an Advanced Access Content System (AACS) licensed by an AACS Licensing Administrator (AACS LA), a consortium including Disney, Microsoft, Matsushita, Sony, and others. The AACS encryption scheme uses Advanced Encryption Standard (AES) encryption and decryption. In AACS title keys utilized for encrypting and decrypting content on the BD are derived from a media key and a random number written on the BD. More particularly, the random number and the media key are used to encrypt/decrypt the title key for authority management. Device keys, assigned to each individual device compliant with the AACS encryption scheme, is utilized for determining the media key from a media key block (MKB) based on a subset difference tree scheme. As the device keys may be unique to each player or recorder, if the device keys are compromised, the AACS LA may update the MKB to revoke the player or recorder. Thus, all releases made after revocation of the player or recorder will become unusable by the player or recorder.
The AACS encryption scheme first appeared in Blu-ray devices in 2006, and since then, some decryption keys have already been determined from software BD players with weak protection and released on the Internet. Typically, a hacker will attempt to gain access to the device keys mentioned above by analyzing memory for presence of the device keys. If the device keys are obtained, and the player, whether software or hardware, is revoked, this may cause inconvenience to the software vendor or hardware manufacturer as they must update the device keys issued with their product each time the device keys are compromised. Likewise, if the player software uses the same device keys for each install, all users of the software must update their software version to be able to play the latest releases. Further, for the player software and recorder software, they would be installed in the same device as the player and a recorder. This further increases the risk of the device keys being exposed.